It is often important and sometimes critical that a back-up power source be available in case a primary power source is either unavailable or degrades until it is not suitable for powering a load. A large computer center may, for example, have access to two or more separate sources of alternating current power for operating some or all of the equipment.
Solid-state transfer switches (SSTS) are normally used in these mission critical environments, such as computer centers, where the loads must remain powered, even when various parts of the electrical system fail. Solid-state transfer switches often include source quality monitoring facilities to automatically initiate a transfer from one source to another in the event that the active source fails. Since the solid-state transfer switch represents the place in the power system where multiple redundant sets of power sources and wiring come together to power the load, the transfer switch is potentially a single point of failure in the system. For that reason, solid-state transfer switches are designed for the utmost reliability and fault tolerance.
These switches utilize pairs of gate activated silicon controlled rectifiers or SCRs that are connected in parallel. Each pair of SCRs conducts current from a single pole of alternating current to a load. One SCR conducts current in one direction and a second, oppositely connected SCR conducts current in a second direction as the AC source switches polarity. When both SCRs are gated into conduction, first one and then the other SCR provides a low resistance path for alternating current power as current flow alternates back and forth during the AC power cycle. A transfer from a faulty power source to an alternate power source requires the active or conductive SCRs be de-activated and a second set of SCRs pairs be activated to couple an alternate power source to the load.
These solid-state transfer switches switch between the sources very rapidly, so that there is minimal effect on the load as it is switched from one source to the other. Because the switches involve no moving part, the switching action is extremely fast, generally much faster than any load could respond to the momentary disruption of power. Furthermore, solid-state transfer switches are carefully designed to insure that at no time the incoming sources be allowed to be connected together. Such a connection, however brief, would have catastrophic consequences for the electrical power system of the facility.
Examples of such solid-state transfer switches and methods of their operation are shown in U.S. Pat. No. 5,555,182, U.S. Pat. No. 5,644,175 and U.S. Pat. No. 5,814,904, each issued to the present inventor.
In most mission critical systems, the secondary power source feeding the SSTS is a single-phase or three-phase voltage source originating at large uninterruptible power systems, engine driven generators or dedicated utility substations. One aspect of these sources is that there is often no guarantee that the phase of the voltage waveforms provided by each source is precisely in phase with the voltage provided by the other source or sources. Although there are often circuits or equipment intended to maintain phase synchronization between the sources, these circuits or equipment represent single points of failure in the system, and tend to be unreliable. The net result is that the sources feeding the SSTS are sometimes out of phase with each other.
A common circuit configuration for mission critical facilities is to distribute electrical power at a higher voltage than the load equipment needs, and step it down to the required voltage at the point of use using a simple transformer. The SSTS is often placed upstream of the transformer in the circuit, i.e., between the transformer and the power sources. This arrangement causes the transformer to be a potential single point of failure, but it is a risk that is often assumed in the interest of economy, and in view of the fact that transformers are generally very reliable devices.
The type of power system described thus far is common in practice, being used in a large number of existing mission critical facilities, although it has a very serious drawback that compromises its overall reliability. Transformers are built of ferromagnetic materials that are subject to saturation if the number of volt-seconds of flux applied to the transformer exceeds a threshold. Volt-seconds of flux are regarded as the time integral of voltage applied to the transformer. Either a small voltage applied for a long time or a large voltage applied for a short time has the same effect, i.e., adding to the total number of volt seconds of flux being supported by the transformer. When the total volt seconds of flux exceeds the capacity of the transfer, the transformer saturates. When a transformer saturates, the impedance of the transformer drops sharply, resulting in the transformer drawing a large amount of current from its power source. This saturation current can cause upstream circuit breakers to trip open, as well as causing the uninterruptible power systems and engine generators to malfunction. Any of these consequences has devastating effects on a mission critical facility.
A power system as described above is vulnerable to failure when a SSTS creates a nearly instantaneous transfer between sources that are out of phase, where the load includes a ferromagnetic transfer. The out-of-phase transfer has the consequence of potentially adding volt-seconds of flux to the transformer far in excess of design limits, initiating a cascade of failures in the power system. Consider the result of a transfer between two sources that are 180° out of phase. If the transfer occurs at the end of one positive half-cycle of voltage, it will be followed by another positive half-cycle of voltage. Two consecutive positive half-cycles of voltage will exceed the transformer flux limit and start the cascade of failures. This disastrous situation has been observed in practice.